Broadband wireless access along with evolving mobile Internet and multimedia services are driving a surge of research and development activities for future wireless communication systems. The Variable Spreading Factor-Orthogonal Frequency and Code Division Multiplexing (VSF-OFCDM) system has been proposed as the most promising downlink wireless access scheme for broadband packet wireless transmission (see H. Atarashi and M. Sawahashi, “Variable spreading factor orthogonal frequency and code division multiplexing (VSF-OFCDM)”, 2001 Third International Workshop on Multi-Carrier Spread Spectrum (MC-SS 2001) & Related Topics, September 2001).
VSF-OFCDM, a variant of the multi-carrier Code Division Multiple Access (CDMA) system, is designed to achieve a very high transmission data rate even in the challenging radio propagation environment with multi-path fading, taking advantage of both multi-carrier and CDMA systems.
In VSF-OFCDM systems, multi-carrier signals can be spread in either the time domain or the frequency domain. It is shown in H. Atarashi, N. Maeda, Y. Kishiyama, and M. Sawahashi, “Performance of VSF-OFCDM with Two-dimensional Spreading Prioritizing Time Domain Spreading,” Technical Report of IEICE, RCS2002-85, June 2002 (in Japanese) that time domain spreading is better than frequency domain spreading in terms of maintaining orthogonality between spread signals. This property makes time domain spreading systems have less inter-code interference, which is crucial in the adaptive modulation and channel coding scheme employed in VSF-OFCDM systems.
VSF-OFCDM systems are designated for downlink transmission where the power consumption, size, weight and cost of mobile receivers are paramount. Conventional VSF-OFCDM receivers work most of the time in the frequency domain and the received time domain signals received from a transmitter have to be transformed to the frequency domain for processing. Although fast algorithms like Fast Fourier Transforms (FFT) are available for the transformation, it is still one of major sources for computational loads, or power consumption in conventional time domain spreading VSF-OFCDM systems where FFT is performed at the chip rate. Therefore, there is a need to simplify the receiver for use in time domain spreading VSF-OFCDM systems.